Since the discovery of massless Dirac transport in graphene, a single atomic layer of hexagonal carbon, there has been significant research activity in the use of graphene as a channel material in transistors. Its superior conductivity and high Fermi velocity (e.g., about 108 cm/s, superior to most semiconductors saturation velocity<2×107 cm/s) make it attractive. However, its zero or small bandgap<0.2 eV for bilayer or trilayer graphene makes it difficult to turn off a graphene channel, limiting the use of graphene to radio frequency (RF) applications, a space where other materials systems (e.g., InGaAs, InAlGaN, etc.) have been optimized and commercialized.
Graphene forms a highly asymmetric Schottky barrier when grown epitaxially on Si-face 4H—SiC. The barrier to holes is about 2.9 eV, while the barrier to electrons is about 0.3 eV. Moreover, this is the only natively grown Schottky material system to date, and has the potential for interfacial engineering to tune the barrier heights systematically through hydrogen intercalation at the SiC/graphene interface. This asymmetric, native tunable Schottky junction is ideal for bipolar-mode operation in semiconductor devices, as proposed by Amemiya et al, and illustrated in FIG. 2. This is due to the action of minority carriers, often ignored in Schottky device operation, an assumption that breaks down for asymmetric wide bandgap materials, whereas we will show experimentally that minority carriers dominate the behavior of these devices.
The injection of minority carriers can lead to conductivity modulation in diodes, as well as gain in a Schottky Emitter bipolar transistor, the subject of this paper. Moreover, the ultrathin graphene has essentially no series resistance, and there is no accumulated charge in the graphene itself. These lead to high frequency operation, and could enable graphene's use in power applications, as the breakdown voltage can now be dominated by the large 2.9 eV Schottky barrier. As a collector, the large p-type Schottky barrier leads to low leakage, enabling efficient charge collection that can also be engineered. Finally, the graphene/SiC material system is ideal for harsh environments due to the high temperature resistance of the materials, as well as the large barriers involved.